A number of possible modulation/multiple access schemes are considered for a physical layer of an ultra wide bandwidth (UWB) communications system by the IEEE 802.15.3a standards working group. One is multiband orthogonal frequency division multiplexing (OFDM).
As shown generally in FIG. 1, an OFDM transmitter sends data (OFDM symbols or ‘blocks’) 101 in parallel on K carrier frequencies (“tones”) 102. Note that for multiband OFDM, all of the carrier frequencies are within one band. For example, in one proposal to the IEEE 802.15 standards working group, the 128 tones are spaced approximately 4 MHz apart, and have a duration of approximately 300 ns. Each of the tones is quaternary phase shift keying (QPSK) modulated.
In practice, OFDM signals are not demodulated by multiple parallel local oscillators for down conversion in an OFDM receiver. Rather, a fast Fourier transform (FFT) of the received signal is equivalent to the signal received on the individual tones. In a practical implementation, the FFT operates on a block of samples of the received signal, e.g., see May et al., “Orthogonal Frequency Division Multiplexing,” Part IV, Molisch (ed.), Wideband Wireless Digital Communications, Prentice-Hall, pp. 309–385, 2001. The FFT is typically implemented as a ‘butterfly’ structure, see, van Nee et al., “OFDM for Wireless Multimedia Communications,” Artech House, pp. 46–51, January 2000.
An alternative, less complex form of signaling is multiband frequency shift keying (FSK), i.e., FSK within each band. Although FSK has usually a lower data rate, and perhaps, a lower quality, FSK signaling is much simpler to implement, reducing the cost of the receiver.
However, providing FSK signaling as an alternative signaling mode to OFDM (so that either OFDM or FSK is allowed as transmit signal) has drawbacks for a standardized implementation. Now, the receiver is required to receive both high complexity OFDM signals and low-complexity FSK signals. That is, the receiver must include two different types of modulators, i.e., both OFDM and FSK. This increases the cost of the receiver.
It is desired to provide FSK reception with an OFDM transceiver without substantially increasing the cost.